Generally, this disclosure relates to forming a continuous microstructured layer by electrolitic deposition in an electrolitic bath.
Electroforming is a process in which a mold is coated with a metal by using the mold as a cathode, to form a metal layer of the desired shape, after which the mold is removed. Suitable metals capable of being deposited by electroforming include nickel, copper, cobalt, iron, gold, silver, platinum, chromium, palladium, rhodium, lead, and the like, and alloys thereof. Nickel is particularly suitable because it fulfills many engineering requirements for physical and mechanical properties, including strength, toughness, and corrosion resistance. Moreover, the physical and mechanical properties of the electroformed component can be varied by changing plating conditions or the composition of the electroforming solution, by alloying with other elements, and by incorporating particles and fibers within the metal matrix, for example.
Where the mold is not expendable (such as by dissolving the mold from the electroformed element), an electroformed element must be removed from the mold. One method of mechanical removal involves the use of impact, such as by a sudden pull or hammer blow. Another method utilizes prying the electroformed element from the mold with a sharp tool. This technique is inexact and can lead to stress marks due to bending in the electroformed element and potentially damage the mold. Another method uses gradual force, such as force applied by a hydraulic ram to push, or force applied by a wheel-puller to pull, the pieces apart.
Other parting methods involve changes in temperature. One method uses differential coefficients of thermal expansion of the electroformed element and mold materials and involves cooling, for example with a mixture of dry ice and acetone. This works best if the mold has a lower coefficient of expansion than the electroformed element. On withdrawal from the cold bath, the electroformed element expands faster than the mold, thereby permitting separation. Another method involves heating, such as with a torch or hot water or oil bath, either to melt or soften a parting compound or to take advantage of a difference in coefficients of expansion between mold and electroformed element.
While changes in temperature may be useful for separating an electroformed element from a mold, such changes may adversely affect dimensional control. In one example, nickel plating is used and is deposited at around 55° C. The mold material is typically copper based, such as brass. Linear thermal expansion coefficients for the mold material and the plating differ: 0.0208 mm/mC for the brass (alloy 280) mold and 0.0133 mm/mC for the nickel plating. After deposition at 55° C., the combined electroformed mold is typically allowed to cool to room temperature, around 22° C. Over a length of 0.5 m, the length change is about 3.75 microns per degree of temperature change and over the 23 degree temperature drop to room temperature, the length change is about 86.25 microns. This is much greater than the acceptable tolerance for controlling positional accuracy in some applications. In addition, the fine pattern in the master tool is typically on a pitch spacing of 200-500 microns and the angled features are susceptible to locking. The term “pitch” refers to the distance between two adjacent features in a piece. Shifting the pattern, relatively, between the master tool and the electroformed element may cause permanent deformation of the features of the electroformed element.